Chem 11

So I did a lab determining the chemical formula of a hydrate. I really just need somebody to check this over for me. We had to find the molecular formula of a hydrate of copper (2) sulphate, CuSO4 .xH2O. My observation table looks like this:

Mass of clean, dry test tube: 21.6g
Mass of test tube + hydrated copper (2) sulphate: 24.1 g
Mass of test tube + anhydrous copper (2) sulphate

The work I did:
Mass of total: 24.1g - 21.6g= 2.5g
Mass of CuSO4: 23.3g- 21.6g= 1.7g

Determine the percent by mass of water in your sample of hydrated copper (2) sulphate. This is what I did:

Percent by mass of water in CuSO4 .xH2O=
2.5g - 1.7g/ 2.5g x100= 32%

Do you expect the mass percent of water that you determined to be similar to the mass percent that other groups determined? Explain.

Not exactly. Other groups could have added more CuSO4 than us or added less. Other groups could have insufficiently heated the test tube and the weight could differ.

a) Calculate the number of moles of H2O:
Molar mass of H2O: 18.02 g/mol
Moles of moles: 2.5g - 1.7g divided by 18.02g/mol = 0.044395 mol of H2O

b) Calculate the number of moles of CuSO4:
1.7g divided by159.62 g/mol (the molar mass of CuSO4) = 0.01065 mol of CuSO4

c) Determine the ratio of moles of H2O of CuSO4 (really kinda sketchy about this one):

0.044395 mol of H2O divided by 0.01065 mol of CuSO4 = 4.16, so approximately 4.

0.01065 mol of CuSO4 divided by 0.01065 mol of CuSO4= 1 mol

So the mole ratio would be 4:1 ?

I really appreciate the help!

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  1. As best I can tell, your work is ok. However, I think you should have gotten closer to CuSO4.5H2O

    What concerns me is the precision of your masses...only two significant digits. I wish your balance would have given when massing four significant digits instead of three.

    See this same experiment writeup.

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  2. Would the answer somehow differ if I had different amounts than the person on the link you posted?

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  3. LYJ

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